The following references are pertinent to the background of the invention and are incorporated herein by reference.
(1) Tadayon et al., "Growth of GaAs at Low Substrate Temperatures, and the Possibility of Zinc Doping at Low Substrate Temperatures", 177th Electrochemical Society Meeting, Montreal, Quebec, Canada (May 6-11, 1990).
(2) Tadayon et al., "A Novel Method for the Growth of Good Quality GaAs at Extremely Low Substrate Temperatures (as Low as 120.degree. C.)", Journal of Vacuum Science and Technology B, Vol. 8, No. 2, 131 (March 1990).
(3) Tadayon, et al., "Reduction of Be Diffusion in GaAs by Migration-Enhanced Epitaxy", Applied Physics Letters, Vol. 55, 59 (1989).
(4) Shtrikman, et al., "High-Mobility Inverted Selectively Doped Heterojunctions", J. Vac. Sci. Technol. B, Vol. 6, 670 (1988).
(5) Tadayon, et al., "Growth of GaAs-Al-GaAs by Migration-Enhanced Epitaxy", Applied Physics Letters, Vol. 53, 2664 (1988).
(6) Kaminska, et al., "Structural Properties of As-rich GaAs Grown by Molecular Beam Epitaxy at Low Temperatures", Applied Physics Letters, Vol. 54, 1881 (1989).
(7) Smith, et al., "New MBE Buffer Used to Eliminate Backgating in GaAs MESFETs", IEEE Electron Device Letters, Vol. 9, 77 (1988).
(8) Horikoshi, et al., "Low Temperature Growth of GaAs and AlAs-GaAs Quantum Well Layers by Modified MBE", Japan J. Appl. Phys., Vol. 25, L868 (1986).
(9) Juang, et al., "Growth and Properties of InAlAs/InGaAs, GaAs:In and InGaAs/GaAs Multilayers", J. Crystal Growth, Vol. 81, 373 (1987).
(10) Missous, et al., "Electrical Properties of Indium Doped GaAs Layers Grown by MBE", J. Crystal Growth, Vol. 81, 314 (1987).
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(12) Tadayon, et al., "Increase in Electrical Activation and Mobility of Si-doped GaAs, Grown at Low Substrate Temperatures, by the Migration-Enchanced Epitaxy Method", J. Appl. Phys. Vol. 67, 589 (1990).
(13) Raiston, J. et al., "Intermixing of Al.sub.x Ga.sup.1-x As/GaAs Superlatices by Pulsed Laser Irradiation", Appl. Phys. Lett., Vol 50, No. 25, p. 1817, Jun. 22, 1987.
Molecular beam epitaxy (MBE) is a highly controlled ultrahigh vacuum deposition process in which thermal beams of molecules or atoms are directed onto a heated substrate. A standard MBE chamber is illustrated in FIG. 1. The MBE chamber is provided with a plurality of furnaces or effusion cells 2 each having a narrow opening 4 which is closed off from or opened to the interior region of the chamber by means of individually controllable shutters 6. The constituents of the various elements desired to be grown are placed in the individual furnaces 2 and heated to vapor conditions. Thermal molecular or atomic beams of these constituents emerge when the shutters are opened, and these beams impinge onto a substrate 10 which is heated to a substrate temperature, T.sub.s, by means of heating element 12. The entire chamber is evacuated to low pressures before and during the beam deposition. The MBE technique permits the growth of highly reproducible and extremely thin epitaxial layers of III-V compounds on large wafer areas. MBE has also been used for group II-VI and IV--IV compounds and for silicon.
Gallium arsenide, GaAs, is a widely used semiconductor material which is grown using MBE techniques. Conventional MBE growth of GaAs is done at substrate temperatures close to 600.degree. C., in an As-rich condition, with a growth rate of about 1 micron per hour, and with the As and Ga shutters open simultaneously.
Semiconductor material such as GaAs are often doped with p-type or n-type materials for use in semiconductor devices. Be is a typical p-type dopant and is used in the MBE technique. However, other materials would be desirable to use in place of Be because Be is extremely toxic. For example, it would be desirable to use Zn to replace Be as a p-type dopant in GaAs. However, Zn has a relatively high vapor pressure and would not be suitable for use in conventional MBE techniques where the substrate temperature is relatively high, e.g., T.sub.s on the order of 600.degree. C., since at such high temperatures Zn will not stick to the GaAs surface.
One way to dope GaAs with a high vapor pressure element (such as zinc), is ion implantation. But ion implantation has three disadvantages:
(1) ion implantation produces damage in the crystal, even after annealing; PA0 (2) the doping profiles are not flat; and PA0 (3) it is impossible to have abrupt doping profiles.
Therefore, it is very desirable to find a method other than ion implantation to grow good quality GaAs and to be able to use a high vapor pressure material, such as Zn, as the dopant.